We have evidence that the rodent in hibernation has two reactions when exposed to low ambient temperatures. Between body temperatures (Tb) of about 3 degrees to 9 degrees C with relatively rapid chilling, the animal may respond at a definite body temperature (Tset) by increasing heart rate, respiratory rate, oxygen consumption, and by muscular activity (Reaction I). When chilled very slowly, a response occurs at a body temperature (Tb) of 1.5 - 2.5 degrees C (Reaction II). In this latter response there is no muscular activity, and threshold is in a very narrow range of Tb. Reaction II will be further defined by obtaining brain temperatures when the reaction occurs, and by observing the effect of preoptic hypothalamic lesions. Since no shivering occurs at the outset, the putative role of brown fat thermogenesis will be explored by: 1) measuring temperature changes in brown fat pads, 2) observing the effect of brown fat ablation, 3) measuring blood flow in the interscapular brown fat pad of the Turkish hamster by cannulating the draining vein. All hibernating rodents become more sensitive to external stimuli as a bout of hibernation continues, but only the Turkish hamster is incapable of arousing on its first day of hibernation. We have shown that arousal will occur if the animal is first infused intraperitoneally with small amounts of epi- or norepinephrine. The effect of catecholamines on progressive irritability will be clarified during a bout by defining precisely the amount of epi- and norepinephrine necessary to produce these results when infused intra-arterially. The change in the concentration of these amines in the blood during a bout of hibernation will be determined.